I recently visited a craft brewery whose menu featured an imperial IPA. By itself, this is quite unremarkable, but what caught my eye was the beer’s description, which proudly boasted IBUs (International Bitterness Units) in excess of 400. Aside from being more than four times the generally accepted saturation level of alpha acids in beer, such a bitterness level, were it actually achievable under normal brewing conditions, would almost certainly render the beer undrinkable.
The IBU is but one part of speech in the language we use to describe beer. To characterize an ale or lager using IBUs exclusively is akin to summarizing a football game using nothing but nouns. And yet, in the seemingly endless race to outdo one another, brewers cling to the IBU as the benchmark for palate intensity, even when the number itself is meaningless. Bitterness is all well and good, but what use is a sports car that can reach 150 miles per hour if the speed limit is 55 mph?
The IBU was developed as an analytical tool to help brewers discuss bitterness with some degree of consistency at a time when sub-optimal storage practices meant that hops could have lost as much as 80 percent of their alpha acids by the time they reached the kettle. The European Brewery Convention and the American Society of Brewing Chemists settled upon the IBU as a common way of describing alpha-acid concentrations. The IBU was never intended to grace marketing copy, but that’s where we find ourselves today.
From a sensory standpoint, knowing a beer’s IBU level is, in fact, somewhat useless without also knowing that beer’s original gravity. Fifty IBUs in a 1.048 Hefeweizen would taste far too bitter, while the same number in a 1.090 imperial stout would leave a syrupy sweetness on the tongue. And the number says nothing about the quality of that bitterness. Is it refined, or is it rough around the edges? Hops aroma and flavor are equally absent from the IBU equation (there are no such things as IAUs—International Aroma Units—or IFUs—International Flavor Units). You simply have to taste it to know.
For the brewer who just wants to make a good beer, a far better sensory indicator is the bitterness-to-gravity ratio (BU:GU ratio). Use brewing software or your favorite bitterness formula (Rager—the oldest—and Tinseth—considered by many the most accurate—are the most common) to estimate IBUs, and then divide that number by the estimated original gravity units.
So, our Hefeweizen and imperial stout above would have ratios of 1.04 (50÷48) and 0.56 (50÷90), respectively. Typically, a Bavarian-style wheat beer should be close to 0.25, and an imperial stout nearer to 0.8, so these numbers tell us that, at least stylistically speaking, our bitterness is probably off (see Ray Daniels’s _Designing Great Beers _for typical ratios for popular styles).
Going back to that alleged 400-IBU imperial IPA, it would need to have an original gravity somewhere around 1.400 to taste balanced (in whatever sense the word is relevant for imperial IPA) because 1.0 is a fairly typical bitterness to gravity ratio for that style. An original gravity beyond 1.100 is very high, and above 1.400 is nonsense.
The IBU is not irrelevant. It tells you something, but it doesn’t tell you everything. When designing your next great recipe, then, remember that IBUs do not by themselves a good beer make. Consider the IBUs, but consider them in context.